1. Field of the Invention
The invention is related to the field of marine seismic exploration.
2. Description of the Prior Art
The use of a slanted cable for gathering marine seismic data has been proposed by Ray et al in U.S. Pat. No. 4,353,121, which issued on Oct. 5, 1982. A primary advantage of using a slanted cable is that it permits reduction, in seismic data, of noise resulting from ghost reflections. In marine data gathering operations, a signal from a seismic source progresses downwardly through the water until it reaches a reflecting interface. The reflected signal then travels upwardly to the seismic detectors positioned along the cable and to the water surface. The signal is then reflected downwardly by the water-air interface and is detected again by the seismic detectors. This reflection is referred to as the ghost reflection. The detection of the ghost reflection along with the primary reflection results in a distorted waveform compared to the waveform of the source impulse. The interrelated effect of the ghost reflection with the primary reflection results in interference cancellation at some frequencies and augmentation at other frequencies.
In the Ray et al disclosure, the cable was deployed at a slope of about two degrees. The primary reflection from each seismic interface and its corresponding ghost reflection is received by each detector and recorded on a field recorder. For each reflecting interface, because of the slope of the cable, the time gap between the detection of the primary and ghost reflections becomes greater, the further the detector is from the source. After typical data processing operations are performed, such as demultiplexing, gain recovery and sorting into common depth point files, static time shifts are applied to correct the primary arrivals to a datum, usually the surface of the water. Then for each interface, the velocity for the primary reflections is determined, the NMO (normal moveout) correction is applied and the primary reflections are time aligned and stacked in the time domain, thereby producing an enhanced primary stack while not emphasizing the individual ghost signals since they are not time aligned.
The data is then processed so that static corrections are also applied to correct the ghost arrivals to datum and the phase of such arrivals is reversed. In a manner similar to stacking of the primary arrivals, the ghost reflections are time aligned and stacked in the time domain, thereby producing an augmented ghost stack while not enhancing the primary stack since they are not time aligned. The two stacks are then summed.
The static time shift method of Ray et al, corrects for time offset of signals traveling vertically. However, this method does not properly correct the time offset of reflections having varying angular orientations in the water. Seismic reflections may reach the seismic cable at varying angular orientations depending on the position of the detector on the seismic cable, the depth of the reflecting interface and the angular dip of the reflecting interface.
It is an object of this invention to provide a method of time shifting all events of data recorded with a slanted cable to make them appear as if recorded by a flat cable.